ZombiU walkthrough, Part Seven: Path to the Royal Bunker
After entering the sewer system you find yourself in a narrow corridor, lit by lamps. Look closely and you'll notice niches in the wall along the way to the end. The one to your left contains a zombie whose swaying head you may notice if you creep close enough. Lure it out and bash its brains in before attempting to cross through.
Enter the sewer tunnels and a zombie will spot you and crawl out of the water to get at you. You'll have plenty of time to prepare, so smack it down when it approaches, then hop down into the water (there's nothing of interest along the sides of the tunnel). Wade through the water a short ways until you can see grating in the distance. Scan here and you'll notice a zombie lurking near the grating. Walk through the water until your character picks up their Backpack and puts it over their head. When you go this far, the zombie will spot you and stalk towards you. Move straight back until your weapon is in view again, then engage the zombie. You can't hit it with your hands on your Backpack.
Use the fallen catwalk to your right to get back on dry land. You'll now be at a crossroads. The right path leads to the Royal Bunker; the left leads to some extra stuff. We'll start by looking at the extra stuff while the area is still safe. Go left across the bridge and follow the path to a door in the rear of the sewer. You'll come out in a small swampy area. Look to your left and move to the tent bathed in red light.
A scan of the area will reveal that it's filled with animals and naught else. There are still many things worth getting, though, if you take the time to search. Follow the wall along the left of the area to find a door leading into the sewers of the Victoria Memorial, which you can unboard and travel freely through. Keep checking along the wall to find a small hole you can crawl through. This leads to an enclosure you spotted on your way through the Victoria Memorial tunnels, and inside a tire is a Firing Speed Upgrade. Stick close to the wall and return to the main swamp area, following the path to the right. This leads to a CCTV junction box you can scan. Backtrack, then check the left wall for another rise out of the water. You'll find debris you can destroy with C4, which you don't have yet, so just mark it on your map. Go through the tunnel beside the debris to find the way back to the sewer.
Take the opposite route across the bridge this time. You'll see a zombie just inside the door; creep up on it and take it out. You're now in a larger room with a central fenced enclosure. Slide under the fence and scan until you see two bodies. These bodies will rise up and attack. Get over them and try to smash one of them, then take out the other. You can also lure them to the gap in the fence and crush their heads as they try to wriggle through. Check the enclosure for Cake and Soda once they're dead. There's also another CCTV junction box on the wall for you to scan.
Despite any more scans you may get of life forms (all rats), the area is clear. Proceed down the catwalks into the dim blue area near the enclosure. This leads to the Buckingham Palace Underground, which will serve as a secondary hub of sorts for the rest of the game.
Wondering about that debris wall? Don't think about it for now. C4 is found as you progress naturally through the story - but it doesn't show up just yet. It's all the more important for you to find the CCTV junction box in Victoria Memorial when you go through the swamp, as it will allow you to pinpoint the debris wall on a return trip with ease. This small swampy area is probably one of the most-overlooked sections of all of ZombiU.